1. Technical Field
The invention is related to plasma reactors for processing semiconductor wafers of the type having a vacuum chamber, an inductive RF coil wound around the upper portion of the chamber and a wafer pedestal which may be capacitively RF powered.
2. Background Art
An inductively coupled RF plasma reactor for processing semiconductor wafers generally includes a vacuum chamber, an inductive RF coil wound around the upper portion of the chamber, a gas supply inlet for furnishing gas to the chamber interior and a wafer pedestal which may be capacitively RF powered. One example is disclosed in U.S. Pat. No. 4,948,458 to Ogle. The upper portion of the chamber including the chamber top has a certain area made of insulator material so that RF power from the RF coil can inductively couple into the plasma inside the chamber. The upper portion of the vacuum chamber may be a cylinder while the top may be a flat disk. The cylinder is an insulator with the inductive RF coil wound around it. The flat top of the chamber may be a conductor.
FIG. 1A illustrates a top portion of a plasma reactor of the type disclosed in European Patent Document No. 0 520 519 A1 and in European Patent Document No. 0 552 491 A1, the disclosures of which are incorporated herein by reference. The plasma reactor of FIG. 1A has a generally cylindrical vacuum chamber 10, a gas inlet 11 on top of the chamber 10 for supplying a gas into the chamber interior from which a plasma is to be ignited, a cylindrical RF coil 12 wound around the exterior of the cylindrical chamber 10, a grounded top conducting lid 14 immediately above the coil 12 and a grounded side wall 16 immediately below the coil 12. The cylindrical RF coil 12 provides RF energy to generate a high density plasma inside the chamber 10 by inductive coupling while the conductive lid 14 is grounded to assure a grounding path for bias RF power (17) applied to a pedestal 18 supporting a semiconductor wafer 20 at the bottom of the chamber 10. One end 12a of the cylindrical coil 12 (e.g., the end nearest the lid) is "hot" because it is connected to the output of an RF source 22, while the opposite end is grounded. In FIG. 6 and FIG. 8 of European Patent Document No. 0 552 491, the coil may be a single section tapped near its center to the RF source.
FIG. 1B illustrates a recent variation from the apparatus of FIG. 1A in which the ceiling 14 is an insulator and both the ceiling 14 and the RF coil 12 have a dome shape, the RF coil 12 being wound in a conical helix to conform with the ceiling 14, as shown in FIG. 1C.
One problem with such a plasma reactor is that the close proximity of the grounded lid 14 to the "hot" end of the cylindrical coil 12 of FIG. 1A or conical helix coil 12 of FIG. 1B permits significant dissipation of the RF power from the coil 12 to the grounded lid 14. Alternatively, if the bottom end of the coil 12 is "hot" and the top end is grounded, then RF power is dissipated from the coil 12 to the grounded side wall 16. As a result, the required RF power level for the cylindrical coil 12 is on the order of 2 to 3 watts, which is comparatively high. Because of the need to ground the lid 14 above the coil 12 and the side wall 16 below the coil 12, such RF power dissipation from the coil 12 has seemed to be unavoidable.
The problem with the inductive RF coil having one end grounded and one end RF powered is that the RF peak-to-peak potential within the coil is very high and this causes strong capacitive coupling of RF power into the plasma. The high electric potential also causes a significant sputtering of chamber interior surfaces. Such sputtering is undesireable because it produces contamination harmful to the processing of the wafer.
The RF potential within the coil also affects capacitive coupling of RF power from the coil to the wafer pedestal through the semiconductor wafer being processed. A high RF potential may cause a high plasma potential so as to increase the rate of physical sputtering of the wafer being processed. It is desired to control the sputtering rate of the wafer being processed by the RF bias power applied to the wafer pedestal. However, such control is distorted to a certain extent by the high capacitive coupling from the RF coil. As employed herein, the term sputtering of the wafer refers to the removal of material from the wafer through the kinetic energy of heavy ions in the plasma colliding with the material being removed. This is different from the removal of material from the wafer by etching. Etching involves a chemical reaction between the material being removed and chemical species in the plasma and is not primarily dependent upon kinetic energy of ions or radicals in the plasma. Sputtering can have beneficial uses in an etch process. For example, in etching a metal such as aluminum, alloy impurities in the aluminum (such as copper) etch very slowly relative to the aluminum and are best removed at the same rate as the aluminum by sputtering. This requires that the RF potential of the wafer pedestal be set to a level such that the sputtering rate of the copper impurity is about the same as the ethc rate of the aluminum, in this example.